The invention relates generally to a thermographic nondestructive testing technique for determining flaws in an object by determining thickness and diffusivity at different points on the surface of the object.
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine cross-sectional thickness of cast metal and other solid objects. Conventionally, the object is probed with ultrasonic waves, which penetrate the surface and are reflected internally at the opposite side or surface of the object. Based upon the time required to receive a reflected wave, the distance to the opposite (back) side can be determined, giving the thickness of the object at that point. Unfortunately, conducting ultrasonic measurements of this sort to examine the cross-sectional thickness would usually necessitate a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface, a stream of liquid couplant must be applied to the surface or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are most often not very practical or even feasible for numerous structural and material reasons. For example, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can require substantial time delays, often of several hours.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Because heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. In contrast to most ultrasonic techniques, a transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one hundred times faster than most conventional ultrasonic methods if testing objects of large surface area.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame, which corresponds to a rectangular area, called a resolution element, on the surface of the object being imaged. Because the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast.
One known contemporary application of transient thermography is to determine the size and relative location (depth) of flaws within solid non-metal composites. Another application of transient thermography is for determining the thickness of metal objects. However, in all known contemporary techniques a calibrated reference standard for thickness is required or temperature dependent images are required to be generated which may intrinsically have greater error than required for accurate analysis.
Therefore, there is a need for a technique that can measure quantitatively, the absolute thickness, diffusivity and depth without using the thickness standards and without dependence on temperature.